This invention relates to spectrophotometers of double beam type, and more particularly, to spectrophotometers of the type wherein an output resulting from the detection of a radiation beam transmitted through a sample material to be analyzed is logarithmically amplified to provide a direct logarithmic representation of the spectral transmittance of the sample material.
As is well known in the art, in the spectrophotometer of double beam type, monochromatic radiation of varying wavelengths is alternately directed to a reference cell and a sample cell containing a sample material to be analyzed to form reference and sample beams which are received by a radiation detector which in turn, produces corresponding electrical outputs, the detector is controlled in a feedback manner such that an output of the detector which responds to the reference beam may be equal to a reference voltage, an output of the detector responding to the sample beam is compared with that of the detector responding to the reference beam at each wavelength, and the ratio of these outputs is derived as the transmittance of the sample.
Among such conventional spectrophotometers, are known spectrophotometers of so-called dynode feedback system using a photodetector in the form of a photomultiplier whose gain is automatically controlled such that outputs of the multiplier which responds to radiation transmitted through a reference cell may be constant at all wavelengths at which measurements are made. One example of these prior art spectrophotometers is shown in FIG. 7.
Referring to FIG. 7, there is illustrated at 1 a main section of a prior art spectrophotometer which includes a radiation source 2 capable of emitting monochromatic radiation of varying wavelengths, for example, a monochromator, a sample chamber or cell 3 containing a sample material to be analyzed, a reference chamber or cell 4 to be described later, a photo detector in the form of a photomultiplier 5, and beam path swithing means 6 for causing monochromatic radiation from the source 2 to alternately enter the sample cell 3 and the reference cell 4 to form sample and reference beams and directing in synchronism the sample and reference beams from the sample and reference cells 3 and 4 alternately to the photomultiplier 5. The reference cell 4 is used in the state that its transmittance is substantially 100% and it shows no characteristic spectral response, that is, in an empty state (an empty cell is placed in the beam path) or in the state that the cell is charged with a standard material having flat spectral response and high transparency. The beam path switching means 6 includes an inlet beam path switching device 7 called a sector adapted to be rotated by means of a motor (not shown) so as to alternately direct the radiation from the source 2 to the sample cell 3 and the reference cell 4 to form sample and reference beams, and an outlet beam path switching device 8 adapted to be rotated in synchronism with the inlet beam path switching device 7 so as to alternately direct the sample and reference beams to the photomultiplier 5. The beam paths extending from the inlet beam path switching device 7 to the outlet beam path switching device 8 through the sample and reference cells 3 and 4 are simply referred to as "sample path" and "reference path", respectively, in this specification. An output of the photomultiplier 5 is supplied to a sample/hold circuit 9 and an error control circuit 10 through an amplifier 11 as will be described in more detail.
The photomultiplier 5 or the amplifier 11 produces output signals S as shown in FIG. 8(A). In the diagram of FIG. 8(A), a represents an impulse corresponding to the reference beam, i.e. beam transmitted through the reference cell 4, and b represents an impulse corresponding to the sample beam, i.e. beam transmitted through the sample cell 3. A low level portion c between these impulses a and b corresponds to background radiation during beam path switching including dark current. The sample/hold circuit 9 is designed to effect sampling in synchronism with a timing pulse TA developed in the duration when the beam path switching means 6 is switched to provide the sample path, that is, the duration of an impulse b as shown in FIG. 8(B). The sample/hold circuit 9 thus produces an output corresponding to the level of an impulse b among output signals S of the amplifier 11, that is, an output correspoding to the intensity of the same beam. Further, the error control circuit 10 functions to derive a signal corresponding to the intensity of the reference beam among output signals S of the amplifier 11, compare it with a reference voltage to determine the difference between them, and control the sensitivity of the photomultiplier 5 in accordance with said difference in a feedback manner such that the impulses a representative of the reference beam intensity among output signals S of the amplifier 11 may be kept at a constant level. In the illustrated example, the error control circuit 10 consists of a circuit 1OA for generating a reference voltage and a synchronization error integrator 1OB adapted to operate in synchronism with a timing pulse TB developed in the duration when the beam path switching means 6 is switched to provide the reference path, that is, the duration of an impulse a as shown in FIG. 8(C), for reading out the level of the impulse a and integrating the difference between said level and the reference voltage. Since the synchronization error integrator 1OB is electrically connected to a high voltage source 12 which drives the photomultiplier 5, the output voltage of the source 12 is controlled by the output of the integrator 10B.
Since the detection system of the spectrophotometer shown in FIG. 7 is controlled such that impulses a among output signals S of the amplifier 11, that is, outputs of the detector which responds to the sample beam are kept at a constant level at all wavelengths, the output of the sample/hold circuit 9 not only corresponds to the intensity of the sample beam, but also directly represents the ratio of the intensity of the sample beam to the intensity of the reference beam at each wavelength, that is, the transmittance of the sample material itself at each wavelength.
The spectral transmittance of a sample material may be represented in two modes, the so-called percentage representation in which the ratio of the intensity of a sample beam to a reference beam is represented in percent, and the so-called logarithmic representation in which the intensity of a sample beam is converted into a logarithmic value and the ratio of the resulting logarithmic value to the intensity of a reference beam is represented. The percentage representation has the advantage that the transmittance of a sample is directly read out. The logarithmic representation is desirable in some cases. That is, where the spectral transmittance of a sample material is considerably low, the logarithmic representation provides more definite recognition of a difference in transmittance.
Radiation detectors, for example, photomultipliers used in conventional spectrophotometers are of current output type whose output current varies with the intensity of radiation. To provide for logarithmic representation in a spectrophotometer using such a detector, it is a common practice in the art to use a current-voltage converting amplifier in combination with a logarithmic amplifier. More specifically, as shown in FIG. 9, a radiation detector in the form of a photomultiplier 5 is series connected to a current-voltage converting amplifier or pre-amplifier 11A and a logarithmic amplifier 11B. A photo current of the photomultiplier 5 is converted and amplified by the pre-amplifier 11A into a voltage which is logarithmically amplified by means of the logarithmic amplifier 1OB. However, such prior art logarithmic representation systems suffer from the drawbacks that noises, drift, offset voltage and other factors induced in the pre-amplifier 11A cause the logarithmically converted data output to contain noise or deleteriously affect the linearity of them. Their influence becomes significant when the radiation transmittance of a sample is low, or an output current of the photo detector is of a small magnitude, resulting in an increased error in measurement. On the contrary, when the sample transmitted beam has a great intensity, the corresponding output of the photo detector will probably be saturated in the pre-amplifier, failing to fully utilize the dynamic range of the photo detector and logarithmic amplifier. It is thus difficult to expand the dynamic range of the entire system to increase measurement accuracy.
To obviate these problems, a spectrophotometer has been proposed in which a photo current of the photo detector is directly supplied to a logarithmic amplifier. However, this type of spectrophotometer is limited to the single monochromatic system which detects only the intensity of a radiation beam transmitted through a sample material and is believed to be applied to the double beam type system only with difficulty. If the above-mentioned logarithmic conversion system is applied without any change to the double beam system in which the detector is controlled in a fedback manner such that an output of the detector responding to a reference beam is equal to a reference voltage, then the detector output responding to the reference beam is also subjected to logarithmic conversion, resulting in the loss of linearity of the detector output responding to the reference beam. This leads to the problem that processing such as correction of the detector output responding to the reference beam and the reference voltage becomes complicated. The double beam system has the advantage that it can cancell the influence of background radiation by subtracting a detector output developed in the duration when the beam path is being switched from one cell to the other cell in the spectrophotometer main section, that is, an output of the detector responding to background radiation (including dark current in the circuit or elements) from an output of the detector which responds to a sample or reference beam, thereby ensuring more accurate analysis. However, the logarithmic convertion of a background radiation output requires more complicated processing.
It is, therefore, an object of the present invention to provide a novel and improved spectrophotometer of the double beam type to which the logarithmic conversion of a detector output is performed into provide a logarithmic representation of the spectral transmittance of a sample material in such a manner that logarithmically converted output data undergo minimal noise influence and have good linearity, and the dynamic range of the measuring system is expanded, thereby significantly improving measurement accuracy over the prior art technique.